The present invention is in the field of glass and ceramic manufacture and particularly relates to vapor deposition methods for the manufacture of glasses and ceramics containing sodium.
In the field of conventional oxide glass manufacture, and particularly for the manufacture of silicate glasses, sodium is one of the oldest and most important fluxes known. As a flux, it acts to aid the melting process and to reduce the softening point of glasses so that they can be melted and formed at lower temperatures.
While sodium is typically present in significant quantities, e.g. 10-20% Na.sub.2 O by weight, in common window and bottle glasses (referred to as soda-lime glasses and consisting mainly of Na.sub.2 O, CaO and SiO.sub.2), it is also an essential minor constituent in numerous other commercial silicate and non-silicate glasses. Examples of glass types containing Na.sub.2 O are low-expansion alkali borosilicate glasses for pharmaceutical and laboratory glassware, alkali borosilicate sealing glasses for joining glass and metal parts, lead silicate glasses for lamp and vacuum tube envelopes, and silicate optical glasses for chemically strengthenable ophthalmic lenses.
Sodium can also be present as a flux in non-silicate oxide glass systems such as in phosphate and germanate glasses. And sodium has been included as an essential constituent in certain pure sodium borosilicate glasses recently utilized for the fabrication of low-loss optical waveguide glasses. For this application, highly pure sodium-containing batch materials which minimize the presence of light absorbing impurities are needed.
In addition to its utility as a flux in oxide glasses, sodium has also been used as a constituent of non-oxide glasses. Thus, for example, in the preparation of halide glasses of high purity for possible use in the manufacture of infrared transmitting optical fibers, sodium fluoride or other sodium halides may be useful as modifiers to control the optical, melting and forming properties of the halide glasses. Again, however, sodium source materials of very high purity will be required.
For specialty glass and ceramic applications, such as in the manufacture of optical fiber glasses, stringent purity requirements favor the utilization of vapor deposition methods for the production of the needed materials. Such methods depend upon the availability of suitable volatile source compounds which can be purified to the requisite degree, thereafter vaporized at temperatures readily maintained in vapor delivery systems and, most preferably maintained at vaporization temperatures for extended periods without decomposition.
Chemical vapor deposition reactions for the commercial production of glasses for optical fibers presently rely on the volatile source compound SiCl.sub.4 and, depending upon the selected composition for the glass, on GeCl.sub.4 and POCl.sub.3, each of which can be efficiently vaporized, stably transported, and rapidly oxidized in the vapor state to the respective oxides SiO.sub.2, GeO.sub.2, and P.sub.2 O.sub.5. However, the demand for improved glass compositions for advanced optical and other specialty glass applications has stimulated work to develop new stable and volatile source compounds for vapor phase glass manufacture. Such manufacture may extend not only to new oxide glasses containing oxides of elements not previously transportable in the vapor phase, but also to halide and other glass compositions in which very high purity is desired.
Sodium is an example of an element which is difficult to supply in the vapor phase using conventional vapor delivery techniques and equipment. U.S. Pat. No. 3,883,336 describes an aerosol technique by which glasses containing sodium and other elements not available as volatile source compounds could be manufactured. According to that method, solutions of non-volatile source materials are introduced into a flame reaction zone where they are oxidized and codeposited with other oxide constituents and subsequently fused to a glass. However, since this method utilizes non-volatile source materials and a carrier solvent, some of the purification advantages of vapor-phase delivery are lost.
U.S. Pat. No. 4,501,602 describes a vapor-phase method for glass manufacture utilizing a variety of organometallic compounds, including a sodium beta-diketonate compound which sublimed at 260.degree. C. However, for vapor-phase glass manufacture wherein glasses containing sodium are to be produced in commercial quantities, even more volatile and stable sodium compounds would be desired.
It is therefore a principle object of the present invention to provide a process for manufacturing pure glass or ceramic products containing sodium, whether oxides, halides, or other forms of this element, wherein the advantages of vapor phase processing can be more effectively realized.
It is a further object of the invention to provide a vapor phase glass manufacturing method adaptable to the manufacture of Na.sub.2 O-containing glasses wherein vaporizable sodium compounds of enhanced volatility and stability are used.
It is a further object of the invention to provide a chemical vapor deposition process for the manufacture of pure fused silica or silica-containing glasses wherein sodium can be introduced into the glass using a volatile organometallic compound of sodium.
Other objects and advantages of the invention will become apparent from the following description thereof.